1. Field of the Invention
This invention relates to a photovoltaic cell having a hetero junction of amorphous silicon carbide and amorphous silicon.
2. Description of the Prior Art
Amorphous silicon is obtained by the plasma decomposition of silane (SiH.sub.4). W. E. Spear et al. discovered in 1976 that the electric conductivity of amorphous silicon could be altered considerably if it was doped with PH.sub.3 or B.sub.2 H.sub.6. D. E. Carlson et al. made a trial solar cell employing amorphous silicon in 1976. These events have drawn much attention to amorphous silicon, and research has come to be actively conducted for improving the conversion efficiency of a thin-film solar cell utilizing amorphous silicon.
The past studies have resulted in the development of thin-film amorphous silicon photovoltaic cells of the Schottky barrier type, p-i-n type, MIS type and hetero junction type. The first three types have been expected to provide high efficiency solar cells. A Schottky barrier type photovoltaic cell made by D. E. Carlson et al. in 1977 showed a conversion efficiency of 5.5%. J. I. B. Wilson et al. made an MIS type photovoltaic cell having a conversion efficiency of 4.8% in 1978. A p-i-n type photovoltaic cell made by Yoshihiro Hamakawa in 1978 achieved a conversion efficiency of 4.5%.
Solar cells of the p-i-n junction type have been unsatisfactory in a number of respects. The p or n type amorphous silicon fails to provide effective carriers having a satisfactorily long life. The p layer has a heavy absorption loss of light, since it has a higher light absorption coefficient than the i layer.
An inverted p-i-n type photovoltaic cell has been proposed to improve the aforesaid drawbacks of the p-i-n type photovoltaic cell. It is a cell in which light is applied to the n type amorphous silicon. It is somewhat better than the p-i-n type cell, since the n layer has a relatively wide band gap. However, the n type amorphous silicon also causes a loss of light absorption to some extent.
It is known to produce amorphous silicon carbide by mixing a hydrocarbon, such as methane and ethane, into silane, and decomposing it by glow discharge. See, for example, D. A. Anderson and W. E. Spear: Phil. Mag., 35, 1 (1977). According to the experiments conducted by D. E. Carlson et al., a solar cell having an intrinsic layer composed of amorphous silicon carbide obtained from silane and methane, and represented by the general formula a-Si.sub.1-x C.sub.x showed a very low efficiency of photovoltaic conversion. While a solar cell having an intrinsic layer formed from silane not containing any methane had a conversion efficiency of 2.27%, the conversion efficiency of a solar cell having an intrinsic layer formed from silane containing 10% of methane was 1.4%, and that of a solar cell having an intrinsic layer formed from silane containing 30% of methane was as low as 0.08%. See, for example, "Topics in Applied Physics," Vol. 36, Amorphous Semiconductors, page 311 (1979), M. H. Brodsky, Springer-Verlag, Berlin, Heidelberg. Thus, it has hitherto been considered undesirable to use methane or any other hydrocarbon as being a defect center in an a-Si semiconductor.